The present invention generally relates to a ring network system. More particularly, the invention is concerned with a configuration control for a ring network including a plurality of line concentrators mutually interconnected through at least one and preferably two transmission lines, in which configuration control is intended to deal with faults possibly occurring in the ring network as well as extension thereof.
In a ring network system, a plurality of line concentrators (also referred to as the wiring concentrators) are interconnected through first and second transmission lines having transmission directions opposite to each other (i.e. duplex or double transmission line path), wherein each of the line concentrators has a plurality of terminal stations (hereinafter referred to also as ST in abbreviation) connected thereto. Signal transmission among the STs is ordinarily conducted through the first ring transmission line serving as the primary transmission line while the second transmission line is employed as an auxiliary or stand-by transmission line and thus may be referred to as the sub-transmission line.
As is disclosed, for example, in an article entitled "Local Area Network in Token Ring System" contained in the monthly periodical "BIT" published by Kyoritsu Shuppan Co. Ltd. in Japan, Vol. 16, No. 3 (1984), there is known a star ring network system in which a plurality of ports are provided on the first ring transmission line within each of the line concentrators, wherein a plurality of the STs are linked to the ports through branch lines, spurs or lobes in a star-like configuration. Although the star ring network system of the configuration mentioned above is advantageous in that the line concentrators and all the terminal stations or STs can be controlled with a single ring control protocol, the system suffers a disadvantage that a fault which occurs in the branch line of the line concentrator or ST provides an obstacle to prevent the data transmission over the whole network system.
To deal with the abovementioned difficulty, each of the line concentrators is provided with a control station terminal or control ST connected to the first ring transmission line at a position most downstream thereof, wherein the control ST is imparted with functions to disconnect the failed port (by forming a bypass) or establish/release a loop-back path between two ring transmission lines. More specifically, each of the STs incorporated in the network system is allocated with an identifier (e.g. port identification number) for identifying the location of the ST in the line concentrator to which the ST is connected, wherein the ST which detects abnormality of the network system is caused to issue an abnormality notification frame (also referred to as beacon frame) including the identifier (port number). Another ST which received the abnormality notification frame relays it to a succeeding ST. In that case, when an ST which is issuing the abnormality notification frame receives a similar abnormality notification frame from other ST located upstream, the former terminates the operation of sending out its own abnormality notification frame or beacon. With this arrangement, only the ST located at a position immediately downstream of the location where fault occurs is not allowed to receive the abnormality notification frame issued by an ST located upstream and thus continues to send out the abnormality notification frame. Each control ST checks the identifier contained in the abnormality notification frame. If the ST issuing the abnormality notification frame is found to be present in the sphere under control of the control ST, the latter performs an operation required to deal with the fault such as, for example, establishment of a bypass for the port suffering the fault.
In Japanese Patent Application No. 60-26236 (Japenese Patent Application Laid-Open No. 187441/1986) corresponding to U.S. patent application No. 828,975 which issued into U.S. Pat. No. 4,777,330, there is disclosed such a network configuration in which each of the line concentrators includes a plurality of first switches for selectively connecting the ports for connection of STs to a first ring transmission line or selectively bypassing the ports and a pair of second switches for short-circuiting the first and second ring transmission lines at the input and output terminals of the associated line concentrator, wherein upon occurrence of fault in an area which is under control of a certain line concentrator, the second switches of that line concentrator are actuated to thereby form a local ring or internal ring which is closed within that line concentrator. With this arrangement, it is possible to determine the location where the fault occurs within the line concentrator by modifying the combination of the ports included in the local ring by means of the first switches while sending a test frame from the control ST to the internal ring to thereby check whether the test frame can circulate along the local ring. If the fault takes place in a branch line connected to a certain port or in an ST connected to the branch line, the second switches can be restored with the associated port remaining in the bypassed state to thereby restore the network to the normal state.
The hitherto known network configuration control system for the ring network system mentioned above is however disadvantageous in that the network system continues to remain in the state containing the fault location with the communication function of the whole ring network system being lost until the location where the fault occurs has been identified and separated or disconnected from the ring network system. More specifically, a lot of time is required to restore the communication from an ST positioned upstream of the fault location to an ST positioned downstream thereof. In other words, the line concentrator suffering the fault exerts adverse influence to the other normal line concentrators, thus giving rise to a problem.
On the other hand, when a fault occurs in a duplex transmission path, the loop-back paths are formed within both the line concentrators positioned on both sides of the location where the fault occurs, whereby communication among the individual STs can be conducted through a loop constituted by the first and second transmission lines. In that case, the network system requires a restoration monitoring function for allowing the network to be restored to the normal state rapidly by releasing the loop-back path upon removal of the fault. As a method of monitoring the restoration from the faulty state, there may be mentioned those disclosed in Japanese Patent Application Laid-Open Nos. 197940/1983 (JP-A-58-197940), 50639/1984 (JP-A-59-50639) and 120633/1985 (JP-A-60-120633). According to these methods, the line concentrator or ST (designated by ST.sup.# A) positioned adjacent to a failed location on one side thereof sends out a sort of monitor signal toward a line concentrator or ST (denoted by ST.sup.# B) positioned adjacent to the failed location on the other side thereof, wherein restoration from the faulty state is detected on the basis of whether the ST.sup.# B could receive the abovementioned monitor signal or not.
According to an open systems interconnection (OSI) model recommended by the International Standard Organization (ISO), the first lower rank layer is physical (PHY) and the second lower rank layer is a data link control (DLC) layer. The DLC layer is classified into further lower levels of a medium access control (MAC) and a logical link control (LLC). In the case of communication at the MAC level, it is required to assure that a token can be normally circulated between two STs, by way of example. In this connection, it is noted that according to the hitherto known restoration monitoring method mentioned above, the reception of the monitor signal is checked in the state in which the first and second transmission lines are independent of each other (i.e. these lines are not linked together in a loop). Consequently, although the reception of the monitor signal at the ST.sup.# B can assure the normality at the PHY level, it can not ensure the normality at the MAC level.